1. Field of the Invention
This invention relates in general to a digital data communication networking, and more particularly to a method and apparatus for measuring the timing difference between physical Inverse Multiplexing for ATM (IMA) links and for delivering a time difference to the IMA layer.
2. Description of Related Art
The demand for large amounts of bandwidth over extended distances is driving interest in networking technologies such as ISDN (Integrated Services Digital Networks), frame relay, SMDS (Switched Multimegabit Digital Service), ATM (Asynchronous Transfer Mode), satellite data communications systems, wireless communications systems, and others. However, to make most of these services universally available requires either a new communications network infrastructure, or significant modifications to the existing one. For example, ATM promises high bandwidth digital connections based upon fixed-size data cells that can carry voice, video and data. But universal ATM also requires that today""s public switched telephone network replace its time-division multiplexed switching fabric with a new ATM switching fabric and enhanced inter-office trunk facilities. Considering that the value of the existing worldwide telephony infrastructure (switches, transmission systems and embedded wiring plant) is estimated to be in the trillions of dollars, it""s unlikely that this infrastructure will be replaced by ATM anytime soon.
While alternative transmission technologies will certainly be implemented over time to handle the growing demand for high-speed digital bandwidth, the existing digital Time-Division Multiplexing (TDM) infrastructure must be fully utilized also. Further, until recently, many end users who wanted to implement ATM WANs were stuck choosing between the high cost of T3 (45 Mbps) or E3 (34 Mbps), and the affordable but inadequate bandwidth of an individual T1 (1.544 Mbps) or E1 (2.048 Mbps).
Two significant enhancements to TDM networking have made possible the full utilization of existing TDM infrastructure while maximizing the utility of the existing worldwide telephony infrastructure. The first is recently-developed software for digital TDM switches that allows dialed connections to exceed the original design channel rate of 56 or 64 kbits, allowing carriers to offer dialed wideband services. The second is the use of specialized equipment which resides at the user""s premises to allow multiple independent digital connections to be xe2x80x9ccombinedxe2x80x9d to create a single, higher-speed end-to-end connection. This technique is known as xe2x80x9cinverse multiplexingxe2x80x9d, and the equipment that performs it is called an inverse multiplexer.
When first introduced, ATM access concentrators were simple multiplexers that aggregated traffic to an ATM uplink. This in itself is a pretty good trick, involving converting traffic to the appropriate ATM Adaptation Layer and assigning priorities to various traffic streams. However, access concentrators now offer a lot more. They feature local switching engines to move traffic between local ports on the box, with more sophisticated traffic management facilities than the previous generation offered.
As mentioned above, a historic problem with large-scale traffic aggregation is the fact that a T1 pipe is often too small to take all of your traffic, but T3 is too large- and too expensive. The latest round of ATM products for the WAN features IMA, which is the UNI (User-to-Network Interface) standard that was ratified by the ATM Forum. IMA can be used over T1 circuits to bridge the broad price and performance gap between T1 and T3 services. With it, trunk capacity can be easily added by simply installing more T1 circuits, up to a maximum of eight or so, beyond which T3 service makes sense.
IMA moves ATM cells across trunks in a cyclic round-robin fashion, so each link is equally loaded. Thus, IMA circuits can provide a measure of fault tolerance, especially when trunks are diversely routed. Diverse routing helps with fault tolerance but can introduce problems of its own. The enemy of IMA is differential delay, a problem that can occur when T1 trunks aren""t routed the same way. IMA must deliver cells in order, so buffers are required to keep traffic moving smoothly. In addition, traditional digital transmission systems and hierarchies have been based on multiplexing signals which are plesiochronous (running at almost the same speed).
When multiplexing a number of 2 Mbps channels they are likely to have been created by different pieces of equipment, each generating a slightly different bit rate. Thus, before these 2 Mbps channels can be bit interleaved they must all be brought up to the same bit rate adding xe2x80x98dummyxe2x80x99 information bits, or xe2x80x98justification bitsxe2x80x99. The justification bits are recognize as such when demultiplexing occurs, and discarded, leaving the original signal. This process is know as plesiochronous operation, i.e., xe2x80x9calmost synchronousxe2x80x9d.
The same problems with synchronization, as described above, occur at every level of the multiplexing hierarchy, so justification bits are added at each stage. The use of plesiochronous operation throughout thee hierarchy has led to adoption of the term Plesiochronous Digital Hierarchy (PDH).
Accordingly, these smaller bandwidth PDH transmission lines may have different clock sources which means different frequencies in these lines. IMA has to compensate this frequency difference by inserting stuffing cells to the lines which have faster clocks. Nevertheless, the IMA specification doesn""t determine any method to be used for measuring the frequency differences between physical links.
It can be seen then that there is a need for a method and apparatus for measuring the timing difference between physical IMA links and for delivering time difference to the IMA layer.
It can also be seen that there is a need for a method and apparatus that uses the timing differences to generate stuffing cells with the proper rate on the transmit links.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for measuring the timing difference between physical IMA links and for delivering time difference to the IMA layer.
The present invention solves the above-described problems by measuring the timing difference between physical IMA links and delivering time difference to the IMA layer. The timing differences is then used to generate stuffing cells with the proper rate on the transmit links. The implementation of the physical interfaces with IMA can be a separate IMA chip and separate physical layer framers. When the IMA chip and framers are attach to each other via a UTOPIA bus which doesn""t carry the line timing, the IMA chip must receive the frequency difference by some other means.
A method in accordance with the principles of the present invention includes obtaining a first data value for use as a reference, obtaining a second data value, processing the first and second data values to obtain an indication of frequency difference and determining a number of stuffing cells to be added to an ATM link based upon the indication of frequency difference.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the first data value includes a reference clock signal and the second data value is a clock obtained from a first ATM link.
Another aspect of the present invention is that the processing further includes examining the reference clock signal and the clock from the first ATM link to obtain the indication of frequency difference.
Another aspect of the present invention is that the processing further includes measuring a phase difference between the reference clock and the clock from the first ATM link and calculating the indication of frequency difference in response thereto.
Another aspect of the present invention is that the calculating further includes computing a time interval for measuring the phase difference and determining the indication of frequency difference according to:             Δ      ⁢              xe2x80x83            ⁢      f        =          ΔΦ              2        ⁢                  xe2x80x83                ⁢        π        *        Δ        ⁢                  xe2x80x83                ⁢        t              ,
where xcex94f is the frequency difference between the reference clock and the clock of the first ATM link, xcex94"PHgr" is the phase difference and xcex94t is the time interval.
Another aspect of the present invention is that the first data value is a counter value of a first buffer containing cells from a first ATM link and the second data value is a counter value of a second buffer containing cells from a second ATM link.
Another aspect of the present invention is that the processing further includes examining the counter value of the first and second counters to obtain the indication of frequency difference.
Another aspect of the present invention is that the processing further includes measuring a cell amount difference based upon the counter value of the first and second counters calculating the indication of frequency difference in response thereto.
Another aspect of the present invention is that the calculating further includes computing a time interval for measuring the cell amount difference and determining the indication of frequency difference according to:             Δ      ⁢              xe2x80x83            ⁢      c        =                            n          1                -                  n          2                            Δ        ⁢                  xe2x80x83                ⁢        t              ,
where xcex94c is the frequency difference between the cells in the buffer for the first ATM link and the cells in the buffer for the second ATM link, n1 is the cell amount of the first ATM link, n2 is the cell amount of the second ATM link and xcex94t is the time interval.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.